A finite element modelling strategy for suture anchor devices

Abstract

Suture or bone anchors are used to reattach a tendon or ligament after it has been torn away from the bone. Anchors provide secure attachments to bone during trauma or reconstructive surgery, holding the ligament or tendon in place and potentially allowing greater mobility during recovery. Computer modelling techniques are used to investigate both established bone anchor technology, such as threaded implants, and emerging technologies such as cement augmentation or sonic-fusion. Sonic fusion is an ultrasound-assisted anchoring method which has recently been introduced in low load maxillofacial applications, and is expected to be used in other low load applications such as hallux valgus alignment procedures and suture attachment. Threaded anchors were examined using two Finite Element (FE) models of human cancellous bone, representing both “normal” and “weaker” bone. Simulation and analysis revealed the critical nature of modelling the microstructure of bone. Changing the direction of loading in the model leads to significant changes in the response of the construct, and this cannot be represented in continuum models, or in physical models using artificial cancellous bone. Rapid prototyping (RP) using 3d printing was used for validation of the FE models. While this method has previously been implemented to create physical bone models, testing an assembly model and comparing it to FE results for inclined loading had not been attempted. RP models were created of the threaded anchor in both “normal” and “weaker” bone, and a sonic fusion model in the normal bone was also created. These models were then subjected to mechanical testing. Results produced from the simulation correlated with the physical results. The importance of a cortical layer was re-confirmed. At the apparent densities simulated, engagement with the cortical layer increases pull-out force dramatically. Engaging the anchor even with a thin cortical layer can produce a significant improvement to pull-out strength. Novel sonic fusion FE models were created from a CT scan of animal bone, and the geometry for both the sonic-fusion pin and bone were taken from the CT scan. Computer generated geometry was used to build pin concepts of varying shapes. It was shown that if good engagement is made with bone, as in the case of all of the concepts created, then sonic fusion can produce a good holding power - comparable with that of a threaded anchor. The results showed that sonic-fusion requires less drill penetration into the bone, meaning less of the inherent bone structure is removed – vital for patients with poor bone quality. Bone cement models were investigated. Bone augmentation models were created, and the addition of cement demonstrated an improvement in anchor holding power. The research showed that there are benefits to using FEA as a tool to evaluate the mechanical aspects of cement distribution. The results proved the hypothesis that augmentation will likely increase the holding power of anchor, and its distribution will affect pull-out significantly. This work has created a method for modelling and evaluating both established and novel bone anchor technology in CT bone geometry, a procedure which could be expanded to other bone implants. It has been validated using the innovative approach of rapid prototyping.